Passive RF components, such as inductors, are used in a variety of important applications in microwave and wireless telecommunications circuits. Illustrative applications for low loss inductors include reactive impedance matching to cancel parasitic capacitance and use as frequency determining elements in filters and oscillators.
In order to reduce costs and to improve reliability, it is desirable, to the extent possible, to provide monolithically-integrated implementations of such microwave and telecommunications circuits. At present, a widely recognized goal in telecommunications is the fabrication of a "single-chip" radio. While many of the components/circuits of a radio can now be readily monolithically integrated into such a single integrated circuit (IC), monolithic integration of low-loss, linear, passive RF components remains problematic.
The difficulties with monolithically integrating low-loss inductors on conventional silicon substrates are well-established. First, inductors fabricated on conducting silicon substrates suitable for forming transistors will be subject to high electrical losses due to the interactions of the inductor's scalar potential with the substrate. A second difficulty involves inductive coupling to the substrate, wherein currents are induced in the substrate due to a vector potential produced by the inductor. Such induced currents degrade inductor performance. Third, dielectric properties of silicon increase parasitic capacitance, thereby lowering the maximum operating frequency of such inductors. And fourth, relatively thick metal layers are required to reduce losses in the inductor structure itself.
As a result of the aforedescribed problems associated with monolithic integration, inductors are typically fabricated "off chip" and assembled either as part of a multi-chip module (MCM) or implemented at the board level as discrete components. Both such approaches involve more assembly steps and more cost than a monolithically-implemented device. Moreover, the parasitic inductance and parasitic capacitance of MCMs or board-level implementations, and the corresponding lack of reproducibility of such parasitic forces, can require that additional functionality (i.e., circuits) is moved off chip. Variable-frequency oscillators (VFOs), for example, typically use discrete off-chip inductors and capacitors for such reasons.
Of late, micro-electromechanical systems (MEMS) technology has been used to address the problems inherent in monolithically integrating low loss, passive RF components. Using MEMS, the functionality of an inductor, and particularly a low loss inductor, can be realized by various implementations of micron-sized electromechanical structures. MEMS-based inductors address the aforedescribed monolithic-integration difficulties by increasing the effective distance between the substrate and the inductor. While some of the MEMS-based, integrable, low-loss inductors that have been proposed to date have effectively addressed the aforedescribed "standard" integration difficulties, they do, Unfortunately, present other problems, as described below.
In a first MEMS-based inductor, the silicon substrate beneath a micron-sized inductor loop is chemically modified in an electrochemical process, or selectively removed, such as by wet etching (commonly referred to as "bulk micromachining"). While difficulties related to the proximity between the inductor coil and the substrate are thereby lessened, the additional processing steps incurred, particularly the required "backside" processing of the silicon wafer, complicate wafer handling and disadvantageously increase production costs. See Von Arx et al., "On-Chip Coils with Integrated Cores for Remote Inductive Powering of Integrated Microsystems," Digest of Tech Papers, 1997 Int'l. Conf. Solid-State Sensors and Actuators (Transducers '97), Chicago, Ill., Jun. 16-19, 1997, pp. 999-1002; Ziaie, et al., "A Generic MicroMachined Silicon Platform for Low-Power, Low Loss Miniature Transceivers," Digest of Tech. Papers, 1997 Int'l. Conf. Solid-State Sensors and Actuators (Transducers '97), Chicago, Ill., Jun. 16-19, 1997, pp. 257-260.
In an alternate approach, a ferromagnetic thin film is deposited under the loop as a core. Such an approach involves additional thin film processing that may prove to be CMOS incompatible. Moreover, such an implementation is not applicable for a device operating at rf frequencies.
In yet a third approach, a MEMS-based inductor is suspended on hinged, polysilicon plates over a silicon substrate. Two or four hinged, micromachined, polysilicon plates are arranged symmetrically about a frame that supports a spiral element (i.e., the inductor coil). A first edge of each plate is hinged to the supporting frame. A remote second edge of each of at least two of those plates ("the driven plates") are hinged to actuators, such as "scratch" drives disposed on the substrate surface. The driven plates are located on opposite sides of the supporting frame. As a voltage is applied to the actuators, they move towards one another, forcing the second edge of each driven plate towards one another. Since the driven plates are hinged at their first end to opposite sides of the frame, said first ends of the driven plates rise as the second edges moves towards one another. As the first ends of the plates rise, the frame and spiral element depending therefrom rise as well. See Fan et al., "Universal MEMS Platforms for Passive RF Components: Suspended Inductors and Variable Capacitors," IEEE Proc. Eleventh Annual Int'l. Conf. MEMS, Jan. 25-29, 1998, Heidelberg, Germany, pp. 29-33.
The aforedescribed suspended inductor suffers from several drawbacks as regards monolithic integration. First, the inductor must be assembled after fabrication hereinafter "actively assembled"), leading to many of the same drawbacks as for a MCM. In other words, after the various plates and structures forming the inductor are fabricated, a separate step must be performed on the working inductor wherein the actuators are energized to suspend the inductor. Second, as the inductance of the final device depends on the gap between the inductor coil and the substrate, either the gap, the inductance, or the circuit performance must be monitored during this active-assembly step. In a commercial process in which a MEMS-based inductor is monolithicaily integrated in a silicon chip, such separate actuation and monitoring steps are impractical and typically unacceptable. Moreover, in the prior art suspended inductor, electrical connection to the inductor coil is disadvantageously made through hinged joints having incomplete metallization.
As such, the art would benefit from a MEMS-based inductor that avoids the drawbacks of the prior art and is readily monolithically integrable into various circuits, such as, for example, wireless telecommunication circuits.